Hiya, kinglurk; welcome!
Not 100% sure about the Ego, as I own the Joye 510, but the empty carts that come installed on the atomizers are just that; empty cartridges.
You can fill them with your choice of filler and vape them as you would any other cartridge.
Most people have abandoned carts (cartridges) in favor of cartos (cartomizers). Less hassle.
I still am on cartridges (modified, but still cartridges). They work for me, as I DIY and try out a lot of flavors.
I will order cartomizers in the future, soon as I find out which are the best.
I'm trying to find out which of them have no fiberfill and don't leak, etc..
So many choices...
Good luck to ya!